1. Field of the Invention
The present invention relates to a rotary driving device having a rotary load body which is driven to rotate integrally about a rotary shaft. The present invention also relates to an image forming apparatus provided with such a rotary driving device.
2. Description of the Related Art
Japanese Laid-open Patent Publication No. 2002-195348 describes an example of a conventional rotary driving device which is applied to an image forming apparatus. The rotary driving device of this Publication includes a rotary body system for rotatably supporting a rotary body, a driving system having an electric motor serving as a rotary driving source for driving the rotary body system to rotate, and a driving/transmission system interconnecting the rotary body system and the driving system.
The rotary body system includes a rotary shaft on which the rotary body is so mounted as to be able to rotate integrally with the rotary shaft and a centrifugal pendulum vibration absorber which is so mounted on the rotary shaft as to be able to rotate integrally therewith. The rotary shaft is rotatably supported by a structural part like a frame via bearings. When the electric motor is actuated, the rotary shaft, the rotary body and the centrifugal pendulum vibration absorber rotate together as a single structure.
The centrifugal pendulum vibration absorber is provided for absorbing vibration of the rotary body which rotates integrally with the rotary shaft on a common axis, and is configured to include a disk which is so mounted on the rotary shaft as to be able to rotate integrally therewith, a plurality of circular holes formed in the disk to pass therethrough at equal intervals along a circumferential direction of the disk and cylindrical pendulums loosely fitted in the individual circular holes.
When the rotary body rotates as a result of actuation of the electric motor in the centrifugal pendulum vibration absorber thus configured, the pendulums loosely fitted in the individual circular holes oscillate therein while producing pendular motion along inner surfaces of the circular holes. Vibrational energy produced by rotation of the rotary body is absorbed by the pendular motion (oscillatory motion) of the pendulums, so that the rotary body is kept from vibrating.
Provided that the aforementioned centrifugal pendulum vibration absorber is so configured that there is a difference L between the radius of each circular hole in the disk and that of each pendulum, the center of the disk is separated from the center of each circular hole by a distance R and the disk rotates at an angular velocity ω, a natural frequency ωn of vibration of each pendulum is known to be given by the following equation:ωn=ω√{square root over (R/L)}
On the other hand, an actual frequency of vibration of the rotary body observed when the rotary body rotates at the angular velocity ω can be determined by calculation or from an experiment. Therefore, it is possible to match the natural frequency of vibration of each pendulum with the frequency of vibration of the rotary body by substituting a value of the actual frequency of vibration of the rotary body for ωn in the aforementioned equation and properly setting values of L and R so that the equation is satisfied. Under conditions where the natural frequency of vibration of each pendulum is matched to the frequency of vibration of the rotary body, vibration produced when the rotary body rotates at the angular velocity ω is effectively absorbed by the oscillatory motion of the pendulums in theory.
In the rotary driving device of Japanese Laid-open Patent Publication No. 2002-195348, however, even if the difference L between the radius of each circular hole in the disk and that of each pendulum and the distance R between the center of the disk and the center of each circular hole are determined based on the aforementioned equation so that the natural frequency ωn of vibration of each pendulum matches the frequency of vibration of the rotary body, and the circular holes are formed in the disk and the pendulums are adjusted according the setting of and L, there can be a case where a desired vibration absorbing effect can not be obtained due to errors in design or manufacture when the rotary body is actually rotated.
Also, if it is found that an appropriate vibration absorbing effect is not obtained after the circular holes are formed in the disk, it may be necessary to manufacture a new disk to achieve an intended result. Making a new disk would however results in increasing cost.